Zeolites are crystalline aluminosilicate molecular sieves which have a microporous three-dimensional framework structure. In general, the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant, ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Zeolites can be represented on an anhydrous basis, by the empirical formula EQU M.sub.2/n O:Al.sub.2 O.sub.3 :XSiO.sub.2
where M is a cation having the valence n, X is generally equal to or greater than 2. In naturally occurring zeolites, M can be Li, Na, Ca, K, Mg and Ba. The M cations are loosely bound to the structure and frequently can be completely or partially replaced with other cations by conventional ion exchange techniques. Currently over 150 species of both naturally occurring and synthetic zeolites are known.
Other crystalline microporous compositions are known which are not zeolitic, i.e., do not contain AlO.sub.2 and SiO.sub.2 tetrahedra as essential framework constituents, but which exhibit the ion-exchange and/or adsorption characteristics of the zeolites. One such group of microporous compositions, i.e., molecular sieves, contain titanium and silicon as the framework elements. For example, U.S. Pat. No. 3,329,481 discloses a crystalline titano-silicate molecular sieve having the x-ray diffraction pattern of pharmacosiderite. U.S. Pat. No. 4,853,202 discloses a titano-silicate molecular sieve having large pores, while U.S. Pat. No. 4,938,939 discloses a small pore titano-silicate molecular sieve. Sandomirskii and Belov, in Sov. Phys. Crystallogr., 24(6), Nov.-Dec. 1979, pp. 686-693 report on the structure of an alkali titanosilicate known as zorite. Sokolova et al. in Sov. Phys. Dokl., 34(7), July 1989, pp. 583-585 disclose the structure of a natural sodium titanosilicate. This mineral has a unique structure related in only one crystallographic direction to the pharmacosiderite structure, with unique structural features in the other two principal crystallographic directions. This new mineral was designated SiTiNaKite by the geologist who originally discovered it in Zap. Vseross Mineral O-va, 121(1), 1992, pp. 94-99. More recently, Poojary, Cahill and Clearfield reported the preparation and structural characterization of a porous titanosilicate with the sitinakite structure in Chem. Mater., 6, 1994, pp. 2364-2368.
One property of these molecular sieves is that they can undergo cation exchange. For example, the alkali metal cations present in these molecular sieves can be exchanged for other metals such as cesium, strontium, mercury, and silver cations. Owing to this property, these molecular sieves can be used to remove various metals from waste streams or may find utility in hydrometallurgical separations of technologically important or precious metals. The effectiveness of any one molecular sieve is determined primarily by its ring or channel diameter, framework charge density and dimensionality of the intracrystalline pores. In particular, the pharmacosiderite structure has a three-dimensional pore structure which offers facile diffusion of cations such as sodium, potassium, strontium, mercury and silver, although the structures' selectivity for certain cations is inferior to that of sitinakite. The sitinakite structure has a one-dimensional pore system which has been shown to display high selectivity for cations such as strontium and cesium but which potentially displays diffusion kinetic limitations due to the low dimensionality of its channel system.
Applicant has synthesized molecular sieves which have a structure which is an intergrowth of the pharmacosiderite and the sitinakite structures. What this means is that these novel molecular sieves display the beneficial ion exchange characteristics of both the pharmacosiderite and sitinakite structures. The molecular sieves of this invention have an empirical formula of EQU A.sub.((4-4x)/n) (M.sub.x Ti.sub.1-z Ge.sub.y).sub.4 (Ge.sub.1-p Si.sub.p).sub.q O.sub.r
where A is an exchangeable cation selected from the group consisting of alkali metals, alkaline earth metals, hydronium ion, ammonium ions, alkylammonium ions having C.sub.1 or C.sub.2 alkyl groups and mixtures thereof, n is the valence of A and has a value of +1 or +2, M is a metal selected from the group consisting of niobium, tantalum, antimony or mixtures thereof, x has a value from about 0.01 to about 0.99, z=x+y, y has a value from 0 to 0.75, p has a value from 0 to about 1, q has a value from about 2.01 to about 2.99 and r has a value from about 14.02 to about 15.98.
It has been found that these novel molecular sieves have good ion exchange properties, especially for cesium ions.